This invention relates to a method and system for treating raw sewage and more particularly to a method and system for treating raw sewage on marine vessels or on mobile land units from such sources as showers, lavatories, kitchens, or, in the case of shipboard applications, the galley, head, laundry, sanitary system and deck drains below water level. Sewage-containing liquid treated in accordance with this invention is rendered suitable for recycling through the various systems from which it came or discharging into streams, lakes or other receptacles. The present invention can be adapted for use in marine vessels, mobile land units, or land base units where space, and therefore compactness, is a significant consideration.
Raw sewage generally consists of both solid waste matter and organic solids dissolved and suspended in liquid. This sewage is high in BOD (Biological Oxygen Demand), coliform bacteria and other potential ecological pollutants. It has been and remains common practice simply to discharge untreated waste and sewage directly into streams, rivers, lakes, etc.
Recently, concern over increasing environmental pollution, especially in navigable waterways and lakes, has led to the enactment of pollution control laws prohibiting the discharge of raw sewage from marine vessels and establishing maximum levels for ecological pollutants in sewage which may be discharged. In addition, it is anticipated that eventually ocean or foreign port dumping of untreated sewage from holding tanks will also be prohibited.
One alternative to direct discharge of sewage is storage of the waste fluids until proper disposal means can be effected. However, this approach is inconvenient and impractical, especially for units which produce great quantities of waste and/or remain far from disposal facilities for extended periods of time.
Another alternative to direct discharge is chemical treatment of the sewage. However, there are obvious chemical storage and supply problems particularly in the case of large vessels remaining at sea for extended lengths of time where large quantities of chemicals are necessitated. In addition, there is danger that the chemicals may not be thoroughly effective without the assistance of comminuting or other ancillary apparatus.
Many current shipboard waste treatment systems employ physical comminutation devices and/or chemical additives, such as in U.S. Pat. No. 3,638,590 to Roberts et al., U.S. Pat. No. 3,395,799 to Kurtz, and U.S. Pat. No. 3,401,115 to Meyer et al. Several problems arise in conjunction with such systems. The macerating or pulping step requires additional equipment, subject to failure or malfunction, which may result in shut down of the entire sewage treatment system. Furthermore, this additional equipment requires maintenance and occupies valuable space. In operation, the comminutor often requires ancillary apparatus in the nature of a coagulant means or supplementary fluid means for maintaining a proper influent consistency. Those requiring chemical treatment are not desirable due to costs and storage problems. Furthermore, many chemicals used to treat waste are pollutants themselves.
Other waste treatment systems such as U.S. Pat. No. 3,625,883 to Valdespino, U.S. Pat. No. 3,276,794 to Andrews, U.S. Pat. No. 3,549,592 to Wiseman, and U.S. Pat. No. 3,487,923 to Visman are undesirable for shipboard or mobile land unit applications because they involve elaborate schemes of storage tanks, intermediate tanks and lagooning tanks which not only occupy substantial space but also require extended periods of time to effect thorough treatment.